The present invention generally relates to adaptive signal processing techniques and, more particularly, to an adaptive processor for a spread spectrum receiver using an adaptive Wiener filter and an adaptive limiter-canceller in conjunction with bypass controls.
A spread spectrum receiver typically extracts transmitted information from a radio frequency signal which consists of the signal with the information, other structured signals (called interference) and atmospheric noise arising from lightning discharges. Adaptive processors cancel the interference. However, in some cases they cancel the signal carrying the information.
The simplest approach to applying adaptive signal processing techniques to the improvement of a VLF spread-spectrum receiver in the presence of strong interfering signals is to implement the techniques within the receiver without the derivation of the interference cancelling signal by itself from an external source. Interference arises from other communication signals being broadcast and from deliberate attempts to prevent reception through the generation of jamming signals.
There are two distinct approaches to cancelling interference. One approach is the use of an adaptive Wiener filter and a time-delayed version of the primary signal as a cancelling signal. This approach only cancels interference consisting of tones. The other approach is the use of an adaptive limiter-canceller and a hard-limited bandpassed version of the primary signal as a cancelling signal. This approach cancels more complicated interference than tones.
Both of these approaches suffer from the drawback that under certain conditions they can cancel the desired signals. However, under certain other conditions both cancel interfering signals which provides significant performance improvements.